The present invention relates generally to antenna systems and, more particularly, to a compact, low-profile patch element suitable for use in airborne or spaceborne phased array antennas.
In the prior art, phased array antennas are favored in many applications, particularly for their versatility. They respond almost instantaneously to beamsteering changes, and they are well-suited for integration into adaptive beamforming systems. A major drawback to the use of phased array antenna systems is their high cost. A typical array may include hundreds or thousands of elements, and each element, with its associated feed and phase shifting circuitry, may cost in the order of thousands of dollars.
One way of reducing the cost per element in a phased array may be through the use of integrated circuit technology, thereby producing a monolithic-like phased array. At the present time, a patch element appears to be a promising candidate for such monolithic implementation.
A patch radiator consists of a conductive plate, or patch, separated from a ground plane by a dielectric medium. When an Rf current is conducted within the cavity formed between the patch and its ground plane, an electric field is excited between the two conductive surfaces. It is the fringe field, between the outer edges of the patch and the ground plane, that launches the usable electromagnetic waves into free space. A low-profile patch radiator, i.e., one in which the thichness of the dielectric medium is typically less than a tenth-wavelength, generates an image patch in a plane under the ground plane which produces a current tending to cancel out the current in the real patch and thereby prevent effective radiation. Patch radiators can, however, be made to operate in a narrow band near their resonant frequency by exciting a high Q, cavity-like mode that effectively couples to the fringe field.
Patch elements are advantageous in phased arrays because they are compact, they can be integrated into a microwave array very conveniently, they support a variety of feed configurations, and they are capable of generating circular polarization. They also have the advantage of cost effective printed circuit manufacture of large arrays of elements. However, because of their need to operate at or very near their resonant frequency, patch elements suffer from the serious disadvantage of narrow bandwidth, typically two to five percent for elements on thin substrates.
The bandwidth of a patch radiator may be increased by the use of a thicker dielectric substrate. However, this practice also reduces the maximum scan angle of the radiator due to surface wave generation. Use of a thicker substrate also adds weight to the radiator, which is a significant problem in airborne applications.
The bandwidth may also be increased by the use of an external matching network for the radiator. In most applications, however, there is insufficient room available for the placement of such a circuit. Dual polarization is especially difficult to achieve in the compact volume of a patch radiator because of the need for a second matching network.